Microbial Nutrition and Growth: Study Guide (Chapter 6)

Microbial Nutrition and Growth

Organic vs. Inorganic Molecules

Understanding the distinction between organic and inorganic molecules is fundamental in microbiology, as it relates to nutrient requirements and metabolic processes.

• Organic molecules: Compounds that contain carbon and hydrogen, often found in living organisms (e.g., carbohydrates, proteins, lipids, nucleic acids).

• Inorganic molecules: Compounds that generally do not contain both carbon and hydrogen together (e.g., water, salts, acids, bases, carbon dioxide).

• Example: Glucose (C6H12O6) is organic; sodium chloride (NaCl) is inorganic.

Chemical Requirements for Microbial Growth

Microorganisms require various nutrients for growth, which can be classified as macronutrients and micronutrients.

• Macronutrients: Required in large amounts (e.g., carbon, nitrogen, oxygen, hydrogen, phosphorus, sulfur).

• Micronutrients (Trace elements): Required in small amounts (e.g., iron, copper, zinc, molybdenum).

• Growth factors: Organic compounds that an organism cannot synthesize and must obtain from the environment (e.g., vitamins, amino acids, purines, pyrimidines).

Nitrogen Fixation

Nitrogen fixation is a critical process for converting atmospheric nitrogen into a usable form for living organisms.

• Definition: The conversion of atmospheric nitrogen gas (N2) into ammonia (NH3), which can be incorporated into organic molecules.

• Carried out by: Certain bacteria and archaea, including free-living and symbiotic species (e.g., Rhizobium).

Oxygen Requirements and Tolerance

Microorganisms are classified based on their oxygen requirements and tolerance.

• Obligate aerobes: Require oxygen for growth.

• Obligate anaerobes: Cannot tolerate oxygen; may lack enzymes to detoxify reactive oxygen species.

• Facultative anaerobes: Can grow with or without oxygen but grow better with oxygen.

• Microaerophiles: Require low levels of oxygen (less than atmospheric concentration).

• Aerotolerant anaerobes: Do not use oxygen but can tolerate its presence.

Enzymes for Oxygen Detoxification

Some enzymes protect cells from toxic oxygen species.

• Catalase: Converts hydrogen peroxide (H2O2) into water and oxygen.

• Superoxide dismutase (SOD): Converts superoxide radicals (O2-) into hydrogen peroxide and oxygen.

• Example: Organisms lacking both catalase and SOD are typically obligate anaerobes.

Temperature and pH Classifications

Microorganisms are classified based on their optimal temperature and pH for growth.

• Temperature classifications:

  • Psychrophiles: Optimal growth at 0–15°C

  • Mesophiles: Optimal growth at 20–45°C

  • Thermophiles: Optimal growth at 45–80°C

  • Hyperthermophiles: Optimal growth above 80°C

• pH classifications:

  • Acidophiles: Grow best at low pH (acidic conditions)

  • Neutrophiles: Grow best at neutral pH (around 7)

  • Alkaliphiles: Grow best at high pH (alkaline conditions)

Cardinal Temperatures

Each microorganism has minimum, optimum, and maximum temperatures for growth, known as cardinal temperatures.

• Minimum temperature: Lowest temperature at which growth occurs.

• Optimum temperature: Temperature at which growth rate is highest.

• Maximum temperature: Highest temperature at which growth is possible.

• Example: If an organism grows between 10°C and 45°C with an optimum at 37°C, it is a mesophile.

Ecological Relationships

Microorganisms interact with each other and their environment in various ways.

• Mutualism: Both organisms benefit.

• Commensalism: One organism benefits, the other is unaffected.

• Parasitism: One organism benefits at the expense of the other.

Biofilms

Biofilms are complex communities of microorganisms attached to surfaces.

• Matrix composition: Primarily polysaccharides, proteins, and DNA (extracellular polymeric substances).

• Significance: Biofilms protect microbes from environmental stress and antibiotics; important in medical and industrial contexts.

Prokaryotic vs. Eukaryotic Growth

Growth in prokaryotes and eukaryotes differs in mechanism and measurement.

• Prokaryotic growth: Increase in cell number via binary fission.

• Eukaryotic growth: Often measured as increase in cell size or complexity.

Measuring Growth in Microorganisms

• Prokaryotes: Growth measured as increase in cell number.

• Eukaryotes (e.g., fungi): Growth may be measured as increase in biomass or size.

Cell Division in Prokaryotes

Prokaryotes reproduce primarily by binary fission.

• Binary fission: A form of asexual reproduction where a cell divides into two identical daughter cells.

• Recognition: Observed as doubling of cell number in cultures.

Phases of Bacterial Growth

Bacterial populations in batch culture exhibit distinct growth phases.

• Lag phase: Cells adapt to new environment; little or no cell division.

• Log (exponential) phase: Rapid cell division; population doubles at a constant rate.

• Stationary phase: Growth rate slows; number of new cells equals number of dying cells.

• Death (decline) phase: Cells die at an exponential rate.

Generation Time and Growth Calculations

Generation time is the time required for a population to double.

• Formula: $N = N_0 \times 2^n$ Where:

  • $N$ = final number of cells

  • $N_0$ = initial number of cells

  • $n$ = number of generations

  $n = \frac{\log(N) - \log(N_0)}{\log(2)}$ Generation time ($g$): $g = \frac{t}{n}$ Where $t$ is the total time elapsed.

• Example: Inoculate with 10 cells at 8:00 am; 2,560 cells at 12:00 pm (4 hours later). $n = \frac{\log(2560) - \log(10)}{\log(2)} = 8$ generationsg = \frac{4\ \text{hours}}{8} = 0.5\ \text{hours} = 30\ \text{minutes}$

Turbidity vs. Colonies

These terms describe different ways to assess microbial growth.

• Turbidity: Cloudiness in a liquid culture due to microbial growth; measured with a spectrophotometer.

• Colonies: Visible masses of cells on solid media, each arising from a single cell or group of cells.

• Usage: Turbidity for estimating cell density in broth; colonies for counting viable cells on plates.

Culture, Inoculum, Specimen, and Incubation

• Culture: Microorganisms grown in a controlled environment.

• Inoculum: Introduction of microbes into culture media.

• Specimen: Sample taken from the environment, patient, or other source for analysis.

• Incubation: Maintaining cultures under conditions suitable for growth.

Aseptic Technique

Aseptic technique prevents contamination of cultures and the environment.

• Importance: Ensures accuracy of experimental results and safety.

• Methods: Sterilizing instruments, using flame, minimizing exposure to air.

Types of Culture Media

• Broth media: Liquid; used for growing large numbers of organisms.

• Plate (solid) media: Contains agar; used for isolating and counting colonies.

• Usage: Broth for propagation; plates for isolation and enumeration.

Preservation of Cultures

Microbial cultures can be preserved by several methods.

• Refrigeration: Short-term storage at 4°C.

• Deep-freezing: Long-term storage at -50°C to -95°C.

• Lyophilization (freeze-drying): Long-term storage by removing water under vacuum.

Measuring Microbial Growth: Direct vs. Indirect Methods

• Direct methods: Count individual cells or colonies (e.g., plate counts, microscopic counts).

• Indirect methods: Estimate cell numbers based on turbidity, metabolic activity, or dry weight.

Components of Plate Media

• Agar: Solidifying agent.

• Nutrients: Peptones, extracts, salts, and sometimes selective or differential agents.

When to Use Different Measurement Methods

• Plate counts: When viable cell number is needed.

• Turbidity: For rapid estimation of cell density in broth.

• Microscopic counts: For direct observation and counting.

Spectrophotometer Function

A spectrophotometer measures the amount of light absorbed or transmitted by a sample to estimate cell density.

• Principle: As cell density increases, turbidity increases, and less light passes through the sample.

• Measurement: Optical density (OD) at a specific wavelength (usually 600 nm).